Next-generation long-haul, fiber-optic communications systems are being designed to operate at 100 gigabits per second over distances of 1,000 kilometers or more. Data are encoded on an optical carrier signal using a quadrature phase shift keying (QPSK) modulator. In QPSK, symbols are represented by one of four points in the I-Q plane. Thus each symbol represents two bits.
For proper operation, data in I and Q channels of a QPSK system are pattern-aligned; i.e. multi-unit-interval skew between I and Q data is reduced or eliminated. In a laboratory setting, deskewing may be performed manually by using external test instrumentation and observing eye diagrams. Skew can vary across power cycles or clock resets, however. It would be convenient if I/Q alignment could be made part of an automatic, self-calibration procedure that could be done at power-up, after clock resets, or whenever desired in the field.
Thus what are needed are methods for automatic I/Q alignment in QPSK systems. The methods should not depend on advanced features of a MUX. Furthermore the methods should be able to correct skew offsets that are less than one unit interval (UI) of the I or Q data streams.